What is weld cracking
What is weld cracking?
Weld cracking is one of the most common serious defects in welded parts. In the joint of welding stress and other embrittlement-causing factors, the welded joint is damaged by the local area of the metal atomic bonding and the formation of a new interface resulting from the gap. It is characterized by a sharp notch and a large aspect ratio. Cracking affects the safe use of welded parts and is a very dangerous process defect. Welding cracks not only occur during the welding process, some have a certain latent period, and some are produced during the reheating process after welding.
Classification of welding cracks
Welding cracks can be classified in different ways according to their location, size, cause of formation and mechanism. According to the conditions of crack formation, can be divided into hot cracks. Cold cracks. Reheat cracks and lamellar tears and other four categories.
Most of them occur at high temperatures close to the solid-phase line and are characterized by distribution along the grain boundary (see interface); however, sometimes they can also form along the “polygonized boundary” at temperatures below the solid-phase line. Thermal cracking usually occurs in the weld metal, but may also form in the weld metal (base material) near the weld fusion line. According to the characteristics of its formation process, it can be divided into the following three cases.
Generated in the weld metal crystallization process at the end of the “brittle temperature” interval, when there is a thin liquid phase layer between the grains, so the metal plasticity is very low, the uneven contraction by cooling and tensile deformation exceeds the allowable value, that is, along the grain boundary liquid layer cracking. The main metallurgical measures to eliminate crystallization cracks are to improve the plasticity of the material in the brittle temperature range by adjusting the composition, refining the grains, and strictly controlling the impurity elements that form low melting point eutectic, etc.; in addition, to minimize the internal tensile deformation in the temperature range from the design and process.
It is mainly produced in the base material near the fusion line of the weld, and sometimes also in the weld channel of multi-layer welding. The formation is due to the local melting along the grain boundary within the metal on the outside of the weld fusion line under the action of welding heat, and the cracking of the liquefied layer along the grain boundary caused by subsequent cooling and shrinkage. Caused by such cracks in two cases: one is the material grain boundaries have more low melting point material; the other is due to rapid heating, so that the decomposition of certain metal compounds and too late to diffuse, resulting in the local grain boundaries of some alloying elements enrichment or even eutectic composition. The principle of preventing such cracks is to strictly control the content of impurities, the reasonable choice of welding materials, to minimize the role of welding heat.
Is formed at a temperature lower than the solid-phase line. It is characterized by the distribution along the “polygonized boundary”, and a crystalline grain boundary is not obvious; easy to produce in single-phase austenite metal. This phenomenon can be explained by the high temperature of welding superheat and unbalanced crystallization conditions, so that the formation of a large number of vacancies and dislocations in the crystal, at a certain temperature. Stress arranged into subgrain boundaries (polygonized grain boundaries), when this grain boundary and harmful impurity-rich areas overlap, often forming microcracks. The method to eliminate such defects is to add alloying elements that can improve the activation of polygonization, such as the addition of W.Mo.Ta in the Ni-Cr alloy; on the other hand, to reduce overheating and welding stress during welding.
According to the main cause can be divided into quenching cracking. Hydrogen delayed cracking and deformation cracking.
Produced in the steel near the martensite transformation point (Ms) (see supercooled austenite transformation diagram) or below 200 ℃ cracking, mainly occurs in the. High-carbon steels, low-alloy high-strength steels and titanium alloys, etc., mainly in the heat-affected zone and within the weld metal. The cracking direction is along the crystal or through the crystal. The main factors for the formation of cold cracking are.
- ① High hydrogen content of the metal.
- ② brittle tissue or tissue that is sensitive to hydrogen embrittlement.
- ③Welding restraint stress (or strain).
Hydrogen-induced delayed cracking
Hydrogen dissolved in the weld metal during the welding process diffuses into the heat affected zone. The concentration of hydrogen, especially in the triaxial tensile stress concentration area, which is prone to cracking, causes hydrogen embrittlement, i.e., reduces the critical stress of the metal at the crack initiation location (or crack front), and causes cracking when the local stress here exceeds this critical stress. The formation of such cracks is characterized by a significant time delay, which is due to the time required for hydrogen diffusion enrichment (incubation period). The conditions for such cracking are the presence of hydrogen and hydrogen-sensitive tissues, and the presence of high confining stresses. Therefore, it often occurs at the root and seam edge of weldments with severe stress concentrations, as well as in the superheated zone. Measures to prevent it include: (i) reducing the hydrogen content in the weld, e.g. by using low-hydrogen electrodes, strict drying of the weld material, etc.; (ii) reasonable preheating and postheating; (iii) using raw materials with low carbon equivalent; (iv) reducing the constraint stress and avoiding stress concentrations (see Hydrogen in Metals).
Hydrogen Delayed Cracking
The formation of this crack is not necessarily due to high hydrogen content, but to the fact that the tensile strain exceeds the plastic deformation capacity of the metal in the case of multilayer welds or fillet welds that produce strain concentrations.
Occurs in certain low-alloyed high-strength steels. Pearlescent heat-resistant steels. Austenitic stainless steel and nickel-based alloys after welding the reheating process at high temperatures. The main reason is generally believed that when reheated to 500-700 ℃ after welding, the superheated zone in the heat-affected zone, due to special carbide precipitation caused by the secondary strengthening of the grain, the precipitation of some weak grain boundary trace elements, as well as the additional deformation of the weld stress relaxation concentrated in the grain boundary, and lead to cracking along the grain. Therefore, this crack has the characteristics of intergranular cracking, and all occur in the heat-affected zone with severe stress concentration in the coarse grain area. In order to prevent this kind of cracking, firstly, materials with low susceptibility to reheat cracking should be selected during design, and secondly, the internal stress and stress concentration problems in the near seam zone should be minimized from the process.
Mainly produced in the thick plate fillet welding, see the attached figure. It is characterized by parallel to the surface of the steel plate and develops in a stepped pattern along the rolling direction. Such cracks are often not limited to the heat-affected zone, but can also appear in the base material away from the surface. The main reason for its generation is due to the laminar distribution of non-metallic inclusions in the metal, making the steel plate plasticity along the plate thickness direction is lower than along the rolling direction, in addition, due to the thick plate fillet welding in the plate thickness direction caused a large welding stress, so caused by laminar tearing. It is generally believed that lamellar sulfide inclusions are the most harmful, while laminar silicates and excessive dense alumina inclusions also have an impact. Prevent this defect, mainly in the metallurgical process should be strictly control the number of inclusions and distribution state. In addition, improve the joint design and welding process, also has a role to play.
Common defects of welding cracks
One of the most common serious defects in welded parts. The weldability of the metal includes two major categories of problems: one is the deterioration of the material properties caused by welding, so that the welded parts lost the original unique properties of the material, such as stainless steel after welding lost its corrosion resistance, etc.; the other is in the welded joint or its vicinity within the base material to produce defects such as cracks and porosity.
The causes of welding cracks and prevention
Welding cracks can be divided into thermal cracks by their nature. Reheat cracks. Cold cracking. Laminar tearing, etc. The following only on the causes of various cracks. Features and prevention methods for specific elaboration.
In welding at high temperatures, so called thermal cracking, which is characterized by cracking along the original austenite grain boundaries.
Depending on the material of the welded metal (low-alloy high-strength steel. Stainless steel. Cast iron. Aluminum alloy and some special metals, etc.), the form of thermal cracking. Temperature interval and the main cause also varies.
At present, thermal cracking is divided into crystalline cracking. Liquefaction cracks and multilateral cracks and other three categories.
1) crystalline cracking is mainly produced in the impurities containing more carbon steel. Low-alloy steel welds (containing S, P, C, Si seam high) and single-phase austenitic steel. Nickel-based alloys and some aluminum alloy welds.
This crack is in the weld crystallization process, in the solid phase near the line, due to the shrinkage of solidified metal, residual liquid metal is insufficient, can not be filled in time, under the stress of cracking along the crystal.
Prevention and cure measures. In metallurgical factors, proper adjustment of the weld metal composition, shorten the range of brittle temperature zone control weld in the sulfur. Phosphorus. Carbon and other harmful impurities; refine the weld metal primary grain, that is, the appropriate addition of Mo. V. Ti. Nb and other elements; in the process, can be preheated by welding. Control the line energy. Reduce the joint constraint and other aspects to prevent.
2) Near seam zone liquefaction crack is a microcrack along the austenite grain boundary cracking, its size is small, occurring in the HAZ near seam zone or interlayer.
Its cause is generally due to welding near the seam zone metal or weld interlayer metal, at high temperatures so that these areas of the austenite grain boundaries on the low-melting eutectic composition is re-melted, in the role of tensile stress along the austenite intergranular cracking and the formation of liquefaction cracks.
This kind of crack prevention measures and crystallization cracking is basically the same.
Especially in metallurgy, as far as possible to reduce sulfur. Phosphorus. Silicon. Boron and other low-melting eutectic content is very effective; in the process, you can reduce the line energy, reduce the concavity of the melt pool fusion line.
3) Polygonization crack is caused by the low plasticity at high temperature during the formation of polygonization.
This kind of crack is not common, its prevention and control measures can be added to the weld to improve the polygonization of the excitation energy of elements such as Mo, W, Ti, etc.
2. Reheat Cracking
Usually occurs in certain steel grades and high-temperature alloys containing precipitation strengthening elements (including low-alloy high-strength steel. Pearlescent heat-resistant steels. Precipitation-reinforced high-temperature alloys, as well as some austenitic stainless steel), they were not found to crack after welding, but in the heat treatment process produced cracks.
Reheat cracking occurs in the welded heat-affected zone of superheated coarse grain parts, the direction is along the fusion line of austenite coarse grain boundary expansion.
Prevention and control of reheat cracking from the selection of materials, you can use fine grain steel.
In the process, the choice of smaller line energy, the choice of higher preheating temperature and with the post-heating measures, the choice of low matching welding materials, to avoid stress concentration.
3. Cold cracking
Mainly occurs in high. Medium carbon steel. Low. Medium alloy steel welding heat affected zone, but some metals, such as some ultra-high strength steel. Titanium and titanium alloys, etc. Sometimes cold cracking also occurs in the weld.
In general, the hardening tendency of steel grades. The amount and distribution of hydrogen in the welded joint, as well as the joint is subjected to the state of the constraint stress is the three main factors that produce cold cracking when welding high-strength steel. After welding the formation of martensite organization under the action of hydrogen elements, with the tensile stress, then the formation of cold cracking.
Its formation is generally through the crystal or along the crystal. Cold cracking is generally classified as toe cracking. Under weld cracking. Root cracking.
Cold crack prevention and control can be from the chemical composition of the workpiece. The choice of welding materials and process measures in three areas.
Should try to use low carbon equivalent materials; welding consumables should be selected with low hydrogen electrodes, welding seam application low strength matching, for high cold cracking tendency of the material can also be used austenitic welding consumables; reasonable control of line energy. Preheating and post-heat treatment is the process measures to prevent cold cracking.
In the welding production due to the use of steel. Welding materials are different, the type of structure. Steel, as well as the specific conditions of construction are different, there may be various forms of cold cracking.
However, the main one often encountered in production is delayed cracking.
There are three forms of delayed cracking as follows.
- 1) Weld toe crack – this crack originates at the junction of the base material and the weld, and has a significant stress concentration area. The direction of the crack is often parallel to the weld path, generally from the surface of the weld toe to the depth of the base material expansion.
- 2) under the weld channel crack – this crack often occurs in the hardening tendency is greater. Higher hydrogen content of the weld heat affected zone. In general, the cracking direction is parallel to the fusion line.
- 3) Root crack – this crack is a more common form of delayed cracking, mainly occurs in the high hydrogen content. Preheating temperature is not enough. This crack is similar to the toe crack and originates in the root of the weld where the stress concentration is greatest. Root cracking may occur in the coarse grain section of the heat affected zone, or in the weld metal.
Hardening tendency of the steel grade. The amount of hydrogen in the welded joint and its distribution, as well as the state of the joint subjected to the constraint stress are the three main factors that produce cold cracking when welding high-strength steel. These three factors are interlinked and mutually reinforcing under certain conditions.
The hardening tendency of steel grades is mainly determined by the chemical composition. Plate thickness. Welding process and cooling conditions. When welding, the greater the hardening tendency of the steel grade, the more likely to produce cracks. Why steel hardening will cause cracking? Can be summarized as the following two aspects.
a. The formation of brittle hard martensite – martensite is carbon in ɑ iron supersaturated solid solution, carbon atoms to gap atoms in the lattice, so that the iron atoms deviated from the equilibrium position, the lattice distortion occurred in a large, resulting in a hardened state of the organization.
Especially in the welding conditions, near the seam zone of the heating temperature is very high, so that the austenite grains have grown seriously, when the rapid cooling, coarse austenite will be transformed into coarse martensite.
From the metal strength theory can be known, martensite is a brittle and hard organization, fracture will consume less energy, therefore, the welded joint with the presence of martensite, crack formation and expansion is easy.
b. Hardening will form more lattice defects – the metal will form a large number of lattice defects under the conditions of thermal imbalance. These lattice defects are mainly vacancies and dislocations.
With the increase in heat stress in the heat-affected zone of welding, under the conditions of stress and thermal imbalance, vacancies and dislocations will move and gather, and when their concentration reaches a certain critical value, a crack source will be formed.
When their concentration reaches a certain critical value, a crack source will be formed. Under the continued action of stress, it will continue to expand and form macroscopic cracks.
Hydrogen is one of the important factors causing cold cracking in high tensile steel welding and has a delayed characteristic, therefore, in many literature, the delayed cracking caused by hydrogen is called “hydrogen cracking”.
Experimental research proves that the higher the hydrogen content of the welded joint of high-strength steel, the greater the susceptibility to cracking, and when the local hydrogen content reaches a certain critical value, cracks begin to appear, and this value is called the critical hydrogen content [H]cr for cracking.
The value of [H]cr for cold cracking varies from steel to steel and is related to the chemical composition of the steel. Steel degree. Preheating temperature, and cooling conditions, etc.
- 1. moisture in the weld material during welding. The rust at the bevel of the welded part. Oil, as well as environmental humidity, etc. are the causes of hydrogen-rich weld seam.
- In general, the amount of hydrogen in the base material and welding wire is very small, while the moisture in the electrode coating and the moisture in the air cannot be ignored and become the main source of hydrogen increase.
- 2. hydrogen in different metal organizations in the dissolution and diffusion ability is different, hydrogen in austenite solubility is much larger than the solubility of ferrite.
- Therefore, in the welding from austenite to ferrite transformation, the hydrogen solubility occurs suddenly drop.
- At the same time, the hydrogen diffusion rate is the opposite, from austenite to ferrite transformation suddenly increased.
In the subsequent cooling and solidification process, due to the rapid decrease in the solubility, hydrogen is strongly escaped, but because of the rapid cooling, so that the hydrogen can not escape and retained in the weld metal to form diffusion of hydrogen.
4. Laminar tearing
An internal low-temperature cracking. Restricted to the base metal of thick plates or weld heat affected zone, mostly occurring in the “L”. “T”. “+” type joints.
It is defined as a stepped cold crack in the base material of the thick steel plate along the thickness direction plasticity is not enough to withstand the welding shrinkage strain in that direction.
Generally due to the thick steel plate in the rolling process, some non-metallic inclusions in the steel rolled parallel to the rolling direction of the strip inclusions, these inclusions caused by the mechanical properties of the steel plate in each guide.
Prevention of laminar tears in the selection of materials can be used in the choice of refined steel, that is, the use of high z performance of the steel plate, but also to improve the design of the joint form, to avoid single side of the weld. Or open a bevel on the side of the z-directional stress.
Laminar tearing and cold cracking is different, it is generated with the steel strength level is not relevant, mainly with the amount of inclusions in the steel and the distribution of the form.
Generally rolled thick steel plates, such as low carbon steel. Low-alloy high-strength steel, and even aluminum alloy plates can also appear lamellar tears. According to the location of laminar tearing can be broadly divided into three categories.
The first category is the formation of laminar tearing induced by cold cracking in the welding heat-affected zone toe or weld root.
The second category is the welding heat-affected zone along the inclusions cracking, is the most common engineering lamellar tear.
The third category away from the heat-affected zone in the base material along the inclusions cracking, generally appear more in the thick plate structure with more MnS sheet inclusions.
The morphology of laminar tearing and the type of inclusions. Shape. Distribution, as well as the location is closely related.
When the rolling direction to the predominantly flaky MnS inclusions, lamellar tearing has a clear step, when the silicate inclusions are dominated by a straight line, such as Al inclusions are dominated by an irregular step.
Thick plate structure welding, especially T and angle joints, under the conditions of rigid constraint, the weld shrinkage will produce a large tensile stress and strain in the direction of the thickness of the base material, when the strain exceeds the plastic deformation capacity of the base metal, inclusions and metal matrix will occur between the separation and microcracking, in the continued action of stress crack tip along the plane where the inclusions to expand, the formation of the so-called “platform”.
There are many factors affecting lamellar tearing, mainly the following.
- 1. the type of non-metallic inclusions. The number and distribution of morphology is the essential cause of lamellar tearing, which is caused by the anisotropy of steel. Mechanical properties of the fundamental differences.
- 2. Z-directional constraint stress Thick-walled welded structures are subjected to different Z-directional constraint stresses during the welding process. Residual stresses and loads after welding, they are the mechanical conditions that cause laminar tearing.
- 3. the effect of hydrogen is generally considered to be near the heat-affected zone, induced by cold cracking into lamellar tears, hydrogen is an important influence.
Since the effect of lamellar tearing is very large and the damage is very serious, it is necessary to judge the sensitivity of steel to lamellar tearing before construction.
Commonly used assessment methods are Z-directional tensile section shrinkage and pin Z-directional critical stress method. In order to prevent lamellar tearing, the section shrinkage rate should not be less than 15%, generally hope = 15-20% is appropriate, when 25%, considered excellent resistance to lamellar tearing.
The main measures to prevent lamellar tears should be taken from the following aspects.
1. Refined steel
The widespread use of iron first desulfurization methods, and vacuum degassing, can be smelted with sulfur only 0.003-0.005% of ultra-low sulfur steel, its cross-sectional shrinkage (Z direction) up to 23-25%.
2. Control the form of sulfide inclusions is to turn MnS into sulfide of other elements, making it difficult to elongate during hot rolling, thus reducing anisotropy. Currently, the widely used additive elements are calcium and rare earth elements. The steel treated above can produce laminated tear-resistant steel plates with a Z-directional section shrinkage of 50-70%.
3. From the perspective of preventing lamellar tears, in the design and construction process is mainly to avoid Z stress and stress concentration, the specific measures are as follows.
- 1) Should try to avoid unilateral welding seam, the use of bilateral welding seam can ease the stress state of the root area of the weld, to prevent stress concentration.
- 2) The use of welding less symmetrical fillet weld instead of welding a large amount of full penetration weld, so as not to produce excessive stress.
- 3) Should be beveled on the side of the Z stress.
- 4) For T-joints, can be pre-welded on the horizontal plate a layer of low-tensile welding material to prevent root cracking, while also easing the welding strain.
- 5) To prevent laminar tearing caused by cold cracking, should try to use some measures to prevent cold cracking, such as reducing the amount of hydrogen. Properly improve the preheat. Control the interlayer temperature, etc.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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